1. Field of the Invention
This invention relates to tobacco smoke filters for use in smoking articles, cigarettes for example.
2. Brief Description of the Prior Art
When a conventional filter tipped cigarette is smoked, the delivery of particulate matter and nicotine in the mainstream smoke increases with each puff. The final puff can deliver two, or even three times more of these smoke components than is delivered in the initial puffs. Proposals have been made for providing filters in the use of which the smoke component delivery rises less steeply during the smoking of a cigarette. Thus, for example, in United Kingdom Patent Specification No. 1,428,018 there are disclosed filters comprising by-pass channels. In use of such a filter, during early puffs smoke passes along the by-pass channel and at the downstream end of the channel passes into a body of filtration material through an orifice which is formed in the otherwise smoke impervious wall of the channel. As smoking proceeds, the orifice becomes blocked by the accumulation of particulate material of the smoke. Eventually, the smoke can no longer pass through the orifice and passes instead through the full length of the body of filtration material. Thus during later puffs the smoke is subjected to a greater degree of filtration than is the case in the earlier puffs. However, the achievement of a desired delivery profile is dependent upon accurate dimensioning of the orifice. At the very high speeds at which filters are required to be produced a consistently accurate formation of a small orifice, as called for in filters according to United Kingdom Patent Specification No. 1,428,018, is very difficult to achieve.
It is an object of the present invention to provide a tobacco smoke filter which, while meeting the requirement for a gradually increasing degree of smoke filtration, is of simple construction and readily makable at speeds consistent with current filter production practice.